Muting system for radio receivers



March 27, 1951 M. G. NICHOLSON, JR., ETAL 2,545,401

MUTING SYSTEM FOR RADIO RECEIQVERS Filed April 24, 1947 Patented Mar. 27, 1951 2,546,401 v v MUTING SYSTEM FOR RADIO REoEIvERs Madison G. Nicholson, Jr., Snyder, and'Maxwell C. Scott, Lancaster, N. Y., assignors, by mesne assignments, to Sylvania Electric Products Inc., New York, N. Y., a corporation of Massachusetts v Application April 24," 1947, Serial No. 743,686

This invention relates to a muting system forA radio receivers, and more particularly to a muting system particularly adapted for use with push button operated' receivers.

y Push button operation of radio receivers is likely*A to produce undesirable noises when the push buttons are operated to change the tuning from one channel to another. Sometimes these noises result from tuning through strong signals whilethe tuning is being changed, and sometimesfrom rebound, rocking, or slight oscilla-y tion of the tuning mechanism just before it reaches its final position. The noise in the latterVv case is particularly bad when the receiver employs a high inertia variable condenser for tuning in conjunction with so-called springloaded push button mechanism.

" It is possible to eliminate this rocking orV bouncing by mechanical means such as dash pot arrangements, but such mechanism is expensive and unduly increases the cost of the receiver.

`Muting of receivers during change of tuning is well known, but in the past the methods of' ruting have been expensive, generally requiring the use of a relay, and such systems often produce undesirable noises which'are frequently'as objectionable as'the noises that are being eliminated. Failures in such muting systems ofte render the entire receiver inoperative.

yIt is an object of this invention to 'provide an electrical muting system which will operate to make`r` the receiver completely quiet during changes of tuning, which will `not introduce noises of its own, and which requires a minimum 'of additional parts, and which parts as are required are inexpensive in themselves and require little or no change in the construction of the receiver in order to be installed.

'.-It is a further object of this invention to pro-v videa muting system which will not interfere with normal operation of the receiver in case of failure of the muting system.

Still. other objects and advantages of our invention will be apparent from the specification. Y The features of novelty which we believe to be characteristic of our invention are set forth with particularity in the appended claims. Our -inventio'n itself, however, both as to its fundamental principles and as to its particular embodiments, will best be understood by reference to thespeciflcation and accompanying drawing,-v

in which the single figure is a circuit diagram showing the muting system in accordance with 7 Claims. (Cl. Z50-20) our invention as 'applied to a push button operated radio receiver.

In accordance with our invention, We bias the grids of the radio and intermediate frequency amplifier tubes negatively to a point beyond cutoff during the time necessary to complete the4 push button tuning operation. This could be;

main closed during the push button tuning operation, Y

However, in the usual radio receiver, there is not suncient negative voltage available to accomplish the muting in the manner just described, and the switch required would be complicated and expensive.

By the invention herein described, we are able to accomplish the muting by the use of the readily available B voltage of the receiver, and a simple switch with a single insulated contact'which is momentarily connected to ground at the start of thev push button tuning operation.

In the drawing herewith, the details of the radio frequency amplifier, converter, intermediate frequency amplier, detector, automatic volume control circuit, and audio amplifier are omitted for simplicity, since they are well understood and are not, per se, part of our invention.

Referring now more particularly to the drawing, I0 designates the antenna on which signals are received and II the tuning mechanism for selecting desired stations, which may include a number of push buttons l2 which are presettable so that upon operation of any button the receiver is tuned to a predetermined channel.

. The tuning mechanism may comprise a gangl condenser, a portion of which is shown as I3.-

Signals are passed in turn through radio frequency amplier and converter III, intermediate frequency amplifier I5, detector and A. V. C. circuit I6, audio amplier I'I, and nally to loud speaker I8. The -B side of the power supply is grounded as usual and is connected to the A. V. C. bus 25 through resistance 2l. The grid returns of the radio frequency and intermediate frequency amplifier tubes are. brought to A..V. C. bus 25.

The -lside of the power supply +B may be connected toA +`B bus 2S, to which the plates and screens of the v'various tubes requiring +B voltage are connected, and through resistance 24 to one side offc'zondenser 23, the other side of which:

3 bus 25 and ground. The common point of condenser 23 and resistor 24 may be connected through resistance 2D and switch I9 to ground.

Switch I9 is diagrammatically indicated as being Operative when any push button I2 is pushed. Preferably switch I9 is arranged as a passing con; tact so that it is closed` only momentarily when any push button is operated. The length of time during which switch I9 is closed may be varied, but normally the time of closure would not be longer than a small fraction of a second, for instance a few milliseconds. Y

The operation of the circuit is as follows; -As long as switch I9 is open, condenser 23 is charged to the voltage of the B supply, the lower plate of 4. following values of resistances and condensers have been found satisfactory:

Although the system as shown is applied to a'V receiver in a manner to mute it when changing the tuning by means of push buttons, it is apparent that it is also applicable to mute a receiver when changing from one wave band to another,

@s by using a Wave change switch.

the condenser being positive and the upper plate negative. When any push buttonl? pushed, switch I9 is closed momentarily. When this happens, the positively charged terminal of con;E denser 23 is connected to ground through resistance 20. This causes condenser 23 to par-` tially discharge into condenser v22, the AJV. by-'pass' condenser.

The magnitude of the negative charge thus transferred to condenser 22 is determined by the ratio of condenser 23 to the .sum of the capacities of ondensers 22 and 23 multiplied 4by the value of the voltage across C23 atthe instant just prior to the closing of switch I 9.

'By proper selection ofthe 'value of capacity 23' in relation to that ofcapacity 22, the desired amount of negative voltage is applied Vto the grid return circuits of the various tubes to carry Vthem past- V'the `cut-olf point on the tube grid voltage v. plate current characteristic and thereby mute the receiver. The rate at which the receiver is restored to normal operation is determined by the values 0f condenser-s 22 and 23 and resistances 2l and 24, representing the total resistance of the A. V. C. system and the diode load resistor ordinarily employed therein to ground.

The length of time the receiver is muted is determined the above two factors; that is, the amount of negative voltage in excess of the tube cut-off points and the rate of discharge of condense'rs f22and 23. Thus, by proper choice of the values of these circuit elements, the length of the muting time can be made just equal or just a little longer than the time required to tune to the"- desired Vchannel yby means of the push button' mechanism. Y

Resistor is included in -fthe'discharge cir-A cuit to prevent excessive ysparlgirig at the contacts of switch T9. This resistor- `also reduces to a negligible value any electrical noise resulting from the closing of the switch. The value of re. sister 20 should not be solar'ge that the charging of condenser 22 from condenser 23 is 'not completed in the time during which the contacts of.-r

In the specification we have explained the principles of our invention and the vbest mode in Whichwe have contemplated applying those principles, so as to distinguish our invention from other inventions; and we have particularltl pointed out and distinctly claimed the part, improvement, or combination which We claim as. our invention or discovery.

While we have shown and described C ain; preferred embodiments of olli" invention, t"wil lf be understood that modioatons and changes" may be made without departing from the spirit,v and scope thereof, as will be clear to those skilled."

in the art.

We claim:

1. In a radio receiver having a plurality of push buttons for station selection, at least one` amplifier tube, and a source of' B voltage, means. for muting said receiver during tuning changesgf. Comprising a condenser interposed between an.' amplifier grid return and ground, a second con-.L denser interposed between the plus B termnal and said grid return, a connection including swltoh. Contacts Connected in series between the plusv B side of sala second condenser and ground. .said switch being provided with passing contacts arranged to be closed momenterlly in response. to operation of a push button to change the llc-.Q ceiver tuning, and a rcSlStance interposed inseries" between plus B termmel and Sold switch.

2. VIn a radio receiver having a plurality'fofL push buttons for station selection, at least one amplifier tube, an automatic volume control line,

to Which the grid return of said amplifier tube` is connected, and a source of B voltage, means: `for muting said receiver during tuning changea.u

comprising a first condenser connected between seid automatic volume control line and ground. e second condenser interposed between the plus series apair of contacts arranged for momentary closure in response to operation of a push button to change the receiver tuning.

3. The combination claimed in claim 2 with ai.

series with said third resistance connected in switch contacts. 4. The combination claimed in i Second resistance` 5.v The combination vclaimed in claim 2 which the time constant of the resistance-con.-

denser circuit is so chosen that the receiver re\ mains muted only substantially 'for the time r`e`" claim 2 'withe third resistance connected in seriesv with .said switch contacts, vthe magnitude of saidthird rex--F sistance being small compared to that of the linterposed in series between said plus B terminal and said switch.

7. In a radio receiver having at least one amplifier tube and a source of B voltage, means for muting said receiver during tuning changes, comprising a condenser connected between an amplier grid return and ground, a second condenser interposed between the plus B terminal and said grid return, and means for applying a momentary ground to the plus B side of said second condenser in response to initiation of a tuning change, and a resistance connected between the plus B terminal and the plus B side of said second condenser.

v MADISON G. NICHOLSON, Jn. MAXWELL C. SCOTT.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,744,935 Van der Pol Jan. 28, 1930 2,133,128 Beggs Oct. 11, 1938 2,135,942 Koch Nov. 8, 1938 2,186,823 De Tar Jan. 9, 1940 2,186,825 Dome Jan. 9, 1940 2,265,394 Ranley Dec. 9, 1941 2,456,952 Kluender Dec. 21, 1948 

